| 摘要 |
For PCM upstream, the analog modem must transmit a filtered sequence of symbols so that the samples of the signal at the input to the codec at the central office (CO) are at a predetermined set of levels. This requires that the analog modem use an 8 kHz symbol rate synchronized to the CO clock. Typical telephone channels have null at DC and 4 kHz. Therefore, for best performance, the signal transmitted by the analog modem should be spectrally shaped to match the channel. The present invention utilizes a generalized Laroia-Tretter-Farvardin (LTF) precoder (FIGURE 2) similar to the one used in V.34. This precoder structure (FIGURE 2) allows spectral shaping and could also allow for constellation shaping gains. Using this precoder structure with spectral shaping has the potential to improve data rates by 3-3.5 kbps.
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